Physicians prefer to treat patients suffering from coronary artery blockage without resort to open heart surgery. This can be accomplished by threading a guide catheter up a large artery to and into the aorta, and down the aorta to the ostium (the opening) of the coronary artery. Next the surgeon inserts a smaller catheter into the guide catheter, and manoeuvers it into and down the coronary artery and its branches to the site of stenosis (arterial blockage). The smaller catheter may have a fine, coiled guide wire in it with which the surgeon can manoeuver the catheter through arterial forks in the network of arteries around the heart. Once the catheter is in place, drugs are injected up the small catheter to treat the stenosis; or a balloon on the end of the catheter is inflated, increasing the arterial lumen available for blood flow (angioplasty).
A surgeon employing this procedure needs to know that he has reached the stenosis which is causing the patient's distress. If the blockage site he has reached is of intermediate severity (10% to 90% obstruction of the lumen), it can be difficult for him to tell if the stenosis he has reached actually impedes blood supply to the heart. Experimental attempts have been made to deal with this problem with a Doppler acoustic transducer fastened to the end of the catheter. The transducer signal, suitably amplified, measures the blood flow velocity through the artery.
A catheter which measures blood flow velocity must be small enough in diameter not itself to restrict the flow of blood through the artery. On the other hand the size limitation makes it difficult to wire the transducer into the catheter. Until the present invention it has not been practical to include a steering wire in the catheter which carries the transducer. Presently available catheters cannot measure flow velocity beyond the coronary artery ostium and are not of significant use in measuring the physiological significance of coronary artery narrowing, or measuring changes in flow velocity in individual coronary arteries. Moreover the location of the transducer on the end of the catheter means that it is subject to occlusion by the arterial wall, so that signal stability over a useful time interval is unreliable.
Hence there exists a need for a catheter thin enough to enter the coronary arteries, having a reliable Doppler acoustic transducer near its distal end, a lumen separated from the conductive wires of the transducer, and a steering wire in the lumen which can be removed so that therapeutic means can be introduced through the catheter to the blockage site.